This application is based upon and claims priority to German Patent Application 199 42 999.5 filed Sep. 9, 1999, and German Patent Application 100 29 175.9 filed Jun. 19, 2000, which applications are herein expressly incorporated by reference.
The invention relates to a method of controlling and automatically controlling the thickness profile in the production of blown film. The invention also relates to a device for carrying out the method.
The thickness profile and especially the circumferential thickness profile of a film tube emerging from a forming tool or annular nozzle of a blown film extruder, in the circumferential position, has thicker and thinner regions. These regions are in largely fixed positions, which adversely affects the winding and further processing of the film tube.
During the production process, to reduce any deviations of the thickness profile as early as possible, hotter regions are stretched to a greater extent and cooler regions to a lesser extent. In prior art devices, the differences in the thickness of the blown film across the circumference are influenced by varying the cooling power across the circumference. The film tube is divided into individual associated circumferential sectors as a function of the number of control elements.
If the cooling power is higher in one circumferential sector, the blown film in that sector cools more quickly. Thus, that sector is stretched to a lesser extent and remains relatively thicker. If the cooling power is lower in one circumferential sector, the blown film retains a higher temperature and can be stretched to a greater extent. As a result, the film thickness is reduced to a greater extent. The differences in film thickness across the circumference are determined by a measuring device. This data is fed into a control device. As a rule, the measuring device is arranged in the direction of production behind a freezing region of the film material.
DE 40 01 287 C2 and EP 0 508 167 A2 disclose cooling air rings. The rings are divided into segments. The volume or the temperature of the cooling air is changed by devices built into the segments.
In DE 40 01 287 A1, to change the individual volume streams, partial streams of the cooling air are branched off and blown off. The partial streams do not participate in cooling the blown film. This is disadvantageous from an energy efficiency viewpoint. The large number of control units fitted directly to the cooling air ring leads to a very tight arrangement of parts.
In EP 0 508 167 A2, a cooling air ring has nozzles in two exit planes. A smaller partial stream is supplied, uncontrolled, to a first plane, relatively close to the annular nozzle of the blow head. A larger partial stream of cooling air is supplied in a second plane in the direction of production. The temperature of the larger partial stream is controlled segment by segment. The heating elements fitted directly in the cooling ring again lead to a tight arrangement inhibiting the flow conditions. The reaction of the heating elements suffers from inertia.
The main disadvantage of the above-mentioned systems is that the devices have to be fitted in the cooling ring. This constitutes interference with the sensitive aerodynamic system of the cooling air ring. Furthermore, because of the influence on the air quantity or temperature in the cooling ring, losses may occur in the overall cooling power.
DE 26 58 518 C2 discloses influencing differences in the film thickness by corrective air nozzles. The nozzles are arranged on the outside above the cooling ring (external air blowing ring). The disadvantage of this device is that the corrective nozzles are arranged above the cooling ring. In this case, thickness corrections can only take place to a limited extent because the film material has already been subjected to cooling air by the main cooling ring. Thus, the temperature of the film material has already been reduced.
It is the stretching process which allows the corrective air to exert its influence. The stretching process has partly already taken place above the main cooling ring. This reduces the influencing potential with respect to thickness corrections in this region. A further disadvantage of this device is that the corrective air nozzles are arranged above the main cooling ring. Thus, due to the space taken up by the corrective air nozzles, there is only very restricted access to the film bubble (when starting the plant), to the nozzle gap (deposits have to be removed regularly from melt exit), and to the main cooling ring (cooling air ring lips to be set as a function of the respective production process). This device has a further disadvantage. Depending on the required end dimension of the film tube, the position of the corrective air nozzles has to be adapted to the film tube diameter behind the main cooling ring.
DE 39 20 194 C2 discloses a method and device to control the thickness profile of a blown film in the course of production. The device uses an additional cooling ring divided into segments. The ring is arranged in the direction of production behind/above a main cooling ring. This means that in this device, the cooling air streams influence the film thickness first flowing downstream into the main cooling air stream. As a result, they only exert a slight influence on the thickness correction of the film tube which has already been cooled.
DE 196 29 076 A1 discloses a cooling ring to cool a film tube emerging from the annular gap of a film blow head. The cooling ring is supplied with cooling air at two different temperatures. Inside the head, the cooling air is variably mixed independently for each segment for thickness controlling purposes. This device requires more sophisticated equipment to generate cooling air with a certain temperature and to mix the cooling air. It also has to have a very compact and complicated design due to the large number of mixing valves closely arranged inside the cooling ring.
DE 44 28 212 A1 discloses a blow head to produce tubular film. Here, an additional cooling ring is arranged between the exit nozzle of the blow head and an uncontrolled main cooling ring. The additional cooling ring includes small pipes. The pipes start from an outer annular line, extend radially inwardly and each pipe contains a heating cartridge to heat the additional cooling air. The cooling air is uncontrolled in the volume stream. The reaction behavior of the heating cartridges is affected by inertia. Thus, the overall control behavior is expected to be bad. The design of the additional cooling ring with the large number of heating cartridges is extremely complicated. Additionally, the cartridges can be adversely affected by dirt. Further, to carry out maintenance work on the heating cartridges, the cooling rings have to be completely dismantled.
A further disadvantage of the above device is that, due to the required differences in temperature, the heating cartridges require a very effective thermal insulation. The cartridges must be insulated relative to each other and relative to the heated annular nozzle of the blow head.
When producing blown film, not only do differences occur in thickness in the circumferential direction, but they also occur in the direction of production. The fluctuations in the direction of production are uniformly superimposed on the differences in circumferential thickness. Thus, the circumferentially determined thickness is subject to periodic fluctuations in the direction of production. The fluctuations have different causes, such as output fluctuations of the extrusion unit or driving fluctuations of the extraction unit which pulls and stretches the tubular film in the longitudinal direction. To minimize the fluctuation causes, gravimetric throughput control systems are provided in the prior art. These systems connect the throughput of raw materials, the conveyor worm speed of the extrusion unit and the driving speed of the extraction unit in a control circuit in order to keep the mean film thickness constant.
A further cause of fluctuations that cannot be compensated for by prior art systems is frequently occurring low-frequency aerodynamic resonance oscillations in the cooling ring system between the blow-out lips of the cooling air nozzles and the tubular film. The film is held by the venturi effect. Also, in the region of the cooling air nozzles, the film still exhibits a highly resilient behavior. The resonance oscillation adversely affects the constancy of longitudinal stretching. Accordingly, this influences the film thickness in the direction of production in the form of a regular vibration. The frequency of the oscillation is within a range smaller than 1 Hz.
It is an object of the present invention to provide a method and device to carry out the method where the above disadvantages of prior art methods and devices are overcome through controllability and a simple design.
The objective is achieved by controlling the thickness profile in the production of blown film. A blown film extruder of the invention has a blow head, a main cooling ring to supply a main cooling gas stream and an additional cooling ring arranged outside the main cooling ring. The additional cooling ring supplies separate additional cooling gas streams. A measuring and controlling device measures the film thickness of the blown film above a freezing zone across the circumference. The additional cooling gas streams are controlled as a function of the measured film thicknesses. The additional cooling gas streams of the film tube are supplied at least from the outside in the direction of production in front of the main cooling gas streams. The streams are individually controlled as a function of the film thicknesses across the circumference with respect to their volume flow rate.
The objective is further achieved by a device to control the thickness profile in the production of blown film which has a blown film extruder with a blow head, a main cooling ring to supply a main cooling gas stream and an additional cooling ring arranged outside the main cooling ring. The additional cooling ring supplies separate additional cooling gas streams. A measuring and controlling device controls the thickness profile of the blown film. The device measures the film thickness across the circumference at the film tube above a freezing zone. The device controls the additional cooling gas streams as a function of the measured film thicknesses. The additional cooling ring is arranged above the blow head of the blown film extruder underneath the main cooling ring. At least one blower or any other air pressure source and a number of volume flow rate control elements and supply lines, which corresponds to the number of additional cooling gas streams, are provided outside the additional cooling ring.
The inventive method is advantageous because it supplies the additional cooling gas streams in the direction of production in front of the main cooling gas stream. Thus, it is possible to ensure that the tubular film is pre-cooled directly after emerging from the annular nozzle of the blow head. The pre-cooling is independent of the cooling effect of the main cooling gas stream. The volume control of the individual additional cooling gas streams is locally separated from the additional cooling ring. The device is uncomplicated, easy to handle and provides a good reaction behavior.
Due to the means for controlling the thickness profile and with the help of additional cooling gas streams, it is possible to provide the main cooling ring with simple adjusting means for optionally providing a plurality of annular gaps in different blow-out planes. This permits optimum adaptation to different process-technological conditions or requirements. It also increases the production output of the device due to a greater cooling effect.
Attaching the additional cooling ring underneath the main cooling ring enables the greatest possible influence on the cooling effect directly at the nozzle exit. The shape and size of the additional cooling ring is independent of the film tube diameter set by the main cooling ring and the inner gas volume of the film bubble. Also, the shape and size of the ring does not restrict access to the film tube, the nozzle gap of the blow head or the main cooling ring. The device to separately control the additional cooling gas streams is positioned outside the additional cooling ring. This is greatly advantageous since the volume flow rate control elements are positioned outside the hot zone of the annular nozzle of the blow head.
Pressurized air is used for the main cooling gas stream to provide cost effectiveness. It is advantageous to use inert gas or air with an inert gas admixture in the additional cooling gas streams. In this way it is possible to avoid oxidation symptoms of the hot film material at the annular nozzle of the blow head. Thus, the development of oxidation products at the annular nozzle can be prevented. In a preferred embodiment, the percentage of additional cooling gas streams is limited to no more than 5% of the entire cooling gas streams.
An improvement in thickness fluctuations in the direction of production can be achieved by, in addition to measuring the thickness distribution in the circumferential direction, taking measurements of the thickness distribution in the direction of production in one or several places on the circumference. Prior to being separated into volume-controllable additional cooling gas streams, a cooling gas stream is generated by an additional blower. The gas stream periodically changes with respect to its total volume flow rate as a function of the measured thickness in the direction of production. Thus, in the direction of production, compensation occurs for otherwise periodically occurring thickness fluctuations.
The volume-controllable additional cooling gas streams obtained by separating a cooling gas stream continuously generated by an additional blower are periodically jointly additionally changed as a function of the measured thickness distribution in the direction of production. Thus, in the direction of production of the blown film, compensation occurs for otherwise periodically occurring thickness fluctuations.
The amplitude and frequency of the uncorrected longitudinal thicknesses are determined by the measurements taken from the film thickness in the direction of production. The fluctuations largely compensate for by counter control of the additional cooling gas streams. Compensation can take place in two ways. One possibility is to control the control elements to generate an oscillation in the process where a corresponding periodic volume flow rate of the separate additional cooling gas streams is obtained. The control of the control elements with respect to frequency and amplitude is set to compensate for the previously determined periodic change in the film thickness in the direction of production. In a second possibility, the cooling gas stream generated by the blower is periodically influenced prior to being separated into individual volume-controllable additional cooling gas streams. The influence can be exerted by a suitable control element in the complete additional cooling gas stream by a flap and/or valve or by changing the speed of the blower.
The device in accordance with the invention is adapted to fit all prior art cooling ring systems. The device can even subsequently be added on. The overall cooling power of the adjoining main cooling ring is in no way adversely affected. Furthermore, there is no interference with the existing main cooling gas system.
According to an advantageous embodiment, the film thicknesses in the circumferential direction are measured intermittently, in terms of time, continuously across the circumference. A measuring head, movable on an annular carrier, scans, in a contact-free way, the thickness across the circumference at certain time intervals. This is possible because the thickness profile across the circumference is always constant with respect to position. The film thicknesses in the direction of production is preferably continuously measured between the individual measurements of the film thicknesses across the circumference in one circumferential position. It is possible to use the same measuring head held in one circumferential position.
It is particularly advantageous if the uncontrolled, circumferentially undivided main cooling gas stream exiting the main cooling ring is supplied in at least two blow-out planes. The cooling effect is clearly increased with a given cooling gas supply output. Depending on the required output rate of the tubular film, it may be sufficient for the circumferentially uniformly distributed main cooling gas and/or the additional cooling gas streams to be blown exclusively on the outside or on the inside against the film tube. Alternatively, it may become necessary for the circumferentially uniformly distributed main cooling gas and/or the additional cooling gas streams to be blown on the outside and on the inside against the film tube.
Further, it is possible to increase the control rate by using additional cooling gas streams on the outside and on the inside. If cooling gas is applied on the outside and on the inside against the film tube, it is advantageous for the blow-out planes of both the main cooling gas stream and of the additional cooling gas streams both on the outside and on the inside to be positioned on substantially corresponding planes.
According to a particularly advantageous embodiment any additional cooling gas streams in corresponding circumferential positions on the outside and on the inside of the tubular film are jointly supplied with pressure and are jointly volume-flow-controlled. The supply lines for additional cooling gas streams, in identical circumferential positions on the outside and on the inside of the film tube, are designed as branch lines of lines with a joint volume flow rate control element. This measure ensures that the design of the pressure supply means and of the control device for the additional cooling gas is kept simple. Further, it allows greater variation with respect to the controllable cooling effect in the individual circumferential regions.
The additional cooling ring may include a one-piece, so-called segment disc of a substantially uniform thickness. The segment disc includes a planar end face provided with supply bores distributed on its outer circumference. Radial grooves open at one end, start from the circumference, and extend substantially as far as the inner circumference. The end face has the radial grooves sealingly resting against a planar counter face of a cover part. The cover part can, especially, be formed by the main cooling ring itself. The underside of the main ring forms the planar counter face against which the segment disc is bolted. The counter face, together with the grooves, forms radial cooling gas channels in the segment disc,. The channels, via individual muffs at the through-bores, are supplied with separately controlled additional cooling gas streams. Near the exit, the individual cooling gas channels can change into one another.
In accordance with the above-mentioned preferred type of cooling gas supply, the main cooling ring includes two annular nozzles. The nozzles are arranged in at least two different planes. The nozzles extend parallel to the wall of the film tube.
An inner additional cooling gas ring to supply separate additional cooling gas streams inside the film tube can be designed in a similar way. The inner ring is supplied in the same way as the first additional cooling ring on the outside. It is also possible to provide a further blower and a number of volume flow rate control elements and further supply lines. These correspond to the number of additional cooling gas streams. However, in a preferred embodiment, the outer and the inner additional cooling rings are connected to branch lines for the additional cooling gas streams in corresponding circumferential positions.
The further additional cooling ring may be a one-piece, so-called segment disc of uniform thickness. The disc has a planar end face and is provided with supply bores distributed on the inner circumference. Radial grooves, open at one end, start from the inner circumference and extend as far as the outer circumference. The end face has the radial grooves sealingly resting against a planar counter face of a cover part. The cover part with the counter face can be formed by an underside of an inner cooling device inside the film tube.
In accordance with the already mentioned type of cooling agent supply, the inner cooling device may have annular nozzles. The annular nozzles are positioned in one or several planes. The annular nozzles are positioned in the planes of the annular nozzles of the main cooling ring. The inner cooling device may also include a centrically arranged air extraction pipe which projects into the film tube. The inner cooling device may have a plurality of annular discs arranged one above the other. Apart from a cover disc, the annular discs may be of identical design.
From the following detailed description, taken in conjunction with the drawings and subjoined claims, other objects and advantages of the present invention will become apparent to those skilled in the art.